The present invention relates to desalination devices such as those which operate on waste heat from internal combustion engines or other sources and, more particularly, to a control system for operating the various elements in a desalination mechanism in accordance with conditions within such a system sensed by the electronic control.
Prior art desalination systems designed to treat nonpotable water, typically sea water, to produce potable water for drinking and other uses have in some instances utilized waste heat from relatively low temperature sources such as the cooling jackets of internal combustion engines. In such an installation the heat source, that is, the engine cooling fluid, is usually below the atmospheric pressure boiling point of sea water or other nonpotable water. Such systems therefore utilize a boiler chamber which is evacuated to a sufficient degree to lower the boiling temperature of the nonpotable water to permit boiling by heat sources in the 120.degree. to 190.degree. F. temperature range. Such systems generally utilize pumped, raw, nonpotable water which is available at temperatures below 100.degree. to cool condensing tubes within the boiler and positioned above a collector. These systems require a fairly sophisticated and delicate balance between the various elements for proper, efficient operation. Thus the amount of heat supplied by the thermal source, the amount of sea water supplied to the condenser, the amount of sea water supplied to the boiler system as well as the amount of fresh water drawn from the collector and the vacuum level within the boiler must all be fairly accurately controlled to maintain balanced system operation.
Particular difficulties with systems of this type are generated at the beginning and end of an evaporating cycle, which difficulties have not been adequately surmounted by prior art systems.
In particular, if the heat source is allowed to operate during times when the boiler vessel is not properly evacuated, for example, hot salt water can scale the surface of the heating elements, resulting in an ultimate reduction of efficiency for the entire desalination system. Similar difficulties occur throughout the system if adequate means are not incorporated for programming the application and removal of various elements during initial system warm-up and ultimate system shutdown. In general, the prior art has attempted to maintain system balance during these periods through a plurality of manually operated controls used in conjunction with a detailed operational procedure in which the system operator activates certain valves after certain time periods have elapsed, the time periods being selected to assure the prevention of damage to system components. Similarly, other prior art systems have utilized time delay circuits for actuating certain system valves at predetermined times so that, even under the most severe conditions, that is, under conditions where the fluid supplying heat to the system is at a relatively high temperature or the raw, nonpotable water is at an extremely high temperature, the system will nevertheless sequence in a manner which will prohibit damage to system components. Each of these systems, in order to prevent system damage, must be designed to accommodate a great variety of temperatures and flow rates depending upon parameters such as the internal combustion engine temperature and sea water temperature which are outside of the system control. As a consequence, when the conditions are not as adverse as might occur in the worst case, system efficiency is greatly reduced by the operation of system components placed in readiness prior to actual vaporization of raw water or collection of fresh water from the system.
These and other disadvantages of the prior art are overcome in the present system through the utilization of plural temperature and vacuum sensors operating in conjunction with a sequencing control system used during warm-up and shutdown of a desalination system. In general, the desalination system includes a boiler containing nonpotable water and a heat exchanger for supplying heat from the cooling fluid of an internal combustion engine to the nonpotable water in the boiler. The flow of fluid from the internal combustion engine cooling jackets is controlled, as by a pump which selectively supplies the fluid to the boiler of the present apparatus or to the normal cooling radiator used for the internal combustion engine. Nonpotable raw water is drawn, typically through a filter, by a second pump and is forced first through a condensing tube located within the boiler to cool the condensing tube to provide fresh water from the evaporated nonpotable water. The raw water is then pumped through a venturi nozzle which is used to provide a vacuum within the boiler of the system to reduce the vaporization temperature of the raw water below that provided by the cooling fluid from the internal combustion engine. A collecting chamber is provided within the boiler below the condensing tubes and fresh water is drawn from this collecting chamber by a third pump which is sufficient to overcome the vacuum within the boiler chamber. This third pump may additionally be used to provide the pressure for a fresh water pressure storage tank. Water may be supplied to the boiler chamber by a connecting line to the condensing tube, this water having been previously warmed in the condensing process and serving as a warm water source for the boiler.
The control system of the present invention, during warmup of the system, includes a manually activated switch for initially activating the raw water pump to pass raw water through the venturi nozzle, thereby reducing the pressure within the boiler chamber and supplying raw water to the boiler chamber. A vacuum sensor is located within the boiler chamber and automatically activates the pump or other control element for applying heated fluid from the thermal source, such as an internal combustion engine, to the water previously admitted to the boiler chamber.
A temperature sensor located in the boiler chamber and responsive to the raw water within that chamber is utilized to control the fresh water pump, drawing water from the condenser collection chamber after the boiler raw water has been raised to a temperature sufficient to begin the evaporation process. In response to this same temperature sensor, a salinity monitoring cell is activated to selectively conduct fluid pumped from the condenser collecting chamber by the fresh water pump to either a storage vessel, if the salinity is below a predetermined level, or back to the boiler chamber, if the salinity does not meet specifications required for drinking water.
When the system is deactivated, the same switch initially utilized for starting the warm-up procedure and starting the raw water pump is opened to automatically deactivate the fresh water pump and the flow control element used to control the heating water fluid from the internal combustion engine. The switch, however, is bypassed so that the raw water pump continues to pump cooling water through the condensing tubes and continues to generate a vacuum within the boiler until a second temperature probe within the boiler indicates that the temperature of the fluid within the boiler are below the evaporation temperature. This temperature probe then automatically controls a bypassing switch so that the raw water pump is deactivated and the system is allowed to reach atmospheric pressure. This sequence therefore assures that the heating tubes within the boiler are not scaled by the application of heated fluids thereto when the system is not in a condition for evaporation, both during warm-up and shutdown of the system.